The invention relates to a metal halide lamp provided with a discharge vessel having a ceramic wall which encloses a discharge space, in which discharge space, which contains Xe and an ionizable filling with NaI and CeI3, two electrodes are arranged whose tips have a mutual interspacing EA, while the discharge vessel has an internal diameter Di at least over the distance EA.
A lamp of the kind mentioned in the opening paragraph is known from WO 98/25294-A (PHN 16.105). The known lamp has a high luminous efficacy and good color properties (among which a general color rendering index Ra of between 40 and 65 and a color temperature Tc of between 2600 and 4000 K) and is highly suitable as a light source for public lighting. The recognition that an acceptable color rendering is possible when Na-halide is used as a filling ingredient of a lamp and a strong widening and reversion of the Na emission in the Na-D lines takes place is utilized in this lamp. This effect requires a high temperature of the coldest spot Tkp in the discharge vessel of, for example, 1170 K (900xc2x0 C.). Inversion and widening of the Na-D lines causes these lines to assume the form of an emission band in the spectrum with two maxima at a mutual interspacing xcex94xcex.
The requirement that Tkp should have a high value excludes the use of quartz or quartz glass for the discharge vessel wall and necessitates the use of a ceramic material for the discharge vessel wall.
A ceramic wall in the present description and conclusions is understood to mean both a wall made of metal oxide, such as, for example, sapphire densely sintered polycrystalline Al2O3 or YAG, and a wall made of metal nitride, for example AIN.
The known lamp not only has an acceptable color rendering but also a very high luminous efficacy. The filling of the discharge vessel for this purpose comprises Ce iodide in addition to Na-halide. The discharge vessel further contains Xe.
A disadvantage of the known lamp is that it has a comparatively wide electrode interspacing and accordingly a very elongate shape, which renders the lamp less suitable for optical applications in which an accurate focusing of the generated light is required.
The invention has for its object to provide a measure by which the above disadvantage is eliminated.
According to the invention, a lamp of the kind mentioned in the opening paragraph is for this purpose characterized in that Dixe2x89xa62 mm, and the relation EA/Di less than 5 is complied with.
The lamp according to the invention has the advantage that the discharge vessel has very compact dimensions which render the lamp highly suitable for use in a headlamp for a motor vehicle. Owing to the small internal diameter in comparison with the electrode spacing, and thus the discharge arc length, the discharge arc is hemmed in by the discharge vessel wall, so that the discharge arc has a sufficiently straight shape for it to be suitable for use as a light source for a motor vehicle headlamp. An internal diameter Dixe2x89xa62 is found to be of essential importance for realizing a sharp beam delineation necessary for use in motor vehicles in combination with a small spot of high brightness immediately adjacent this delineation. Preferably, Dixe2x89xa61.4 mm. Such a very small internal diameter renders the lamp particularly suitable for use as a light source in a complex-shape headlamp. An advantage of such a headlamp is that no separate passing-beam cap is required in the formation of the light beam to be generated in order to realize a sufficiently sharp beam delineation. The Di, however, is chosen to be so great that a minimum switching life of 2000 hours can be realized. Preferably, the relation EA/Di greater than 2.75 is also complied with. It is achieved in this manner that a sufficiently great value for EA can still be realized while retaining sufficiently small dimensions of the optically active source. The lamp is particularly suitable for use in a headlamp with a European passing beam when the internal diameter Di is chosen such that the relation 1.4 less than Dixe2x89xa62 is complied with. A passing-beam cap will generally be used here which intercepts part of the light emitted between the electrode tips such that the beam formed by the lantern avoids dazzling of oncoming traffic.
The optical dimensions of the light source are furthermore favorably influenced by a suitable choice of the wall thickness. This is preferably chosen such that the wall of the ceramic discharge vessel has a thickness of at most 0.4 mm at least over the distance EA. If the lamp serves as a complex-shape lantern, the wall thickness of the discharge vessel will preferably be at most 0.3 mm. Although the ceramic wall material in itself has generally strongly light-scattering properties, a light source is here advantageously realized which has optical dimensions comparable to usual dimensions of existing headlamps fitted with incandescent coils.
It is necessary that sufficiently high concentrations of Na and Ce should be present in the discharge so as to achieve a high luminous efficacy and good color properties, which manifest themselves in the value of xcex94xcex. The value of xcex94xcex depends inter alia on the molar ratio NaI:CeI3 and the level of Tkp. It was found in the lamp according to the invention that a value for xcex94xcex of at least 3 nm is required. Preferably, the value of xcex94xcex is xe2x89xa66 nm.
Further experiments have shown that it is desirable for the discharge vessel of the lamp to have a wall load of xe2x89xa6120 W/cm2. The wall load is defined here as the quotient of the lamp power and the outer surface of that portion of the discharge vessel wall which is situated between the electrode tips. It is achieved thereby that a required high value of xcex94xcex can be realized while at the same time the maximum wall temperature of the discharge vessel remains limited during lamp operation. The temperatures and pressures prevailing in the discharge vessel in the case of wall load values above 120 W/cm2 become such that chemical processes attacking the discharge vessel wall give rise to an unacceptable shortening of lamp life. In addition, thermal stresses in particular resulting from temperature gradients during heating-up after ignition and cooling-down after extinguishing of the lamp form a source of an unacceptable shortening of lamp life.
In an advantageous embodiment of the lamp according to the invention, the discharge vessel is closed off at one end by a ceramic projecting plug, and a portion of the ceramic projecting plug and an adjoining portion of the ceramic discharge vessel are provided with an external coating. This achieves on the one hand a better temperature control and thus a higher temperature of iodide salts in the filling and on the other hand a cutting-off of light which issues behind the electrode tip, which is highly favorable for realizing a sharp beam delineation. Pt is found to be highly suitable as a material for the coating. A further advantage is that blackening of the wall behind the electrode does not affect the lumen output of the lamp. A lamp suitable for a complex-shape lantern is preferably provided with an external coating at both ends. Although a coating at that end of the discharge vessel which is at the lamp cap side could suffice, the provision of the coating at both ends achieves a symmetrical construction of the discharge vessel. This is of major advantage both in the manufacture of the discharge vessel and during subsequent mounting of the lamp. The coating preferably extends over the ceramic discharge vessel up to at least 0.5 mm from the electrode tip. On the other hand, the coating preferably does not extend beyond the electrode tip, since this would adversely affect the lumen output of the lamp.
According to the invention, the molar ratio NaI:CeI3 lies between 2 and 25. It is found on the one hand that the luminous efficacy becomes unacceptably low and on the other hand that the light radiated by the lamp contains an excess quantity of green in the case of a ratio below 2. A correction of the light color, for example through the addition of salts to the ionizable filling of the discharge vessel, is only possible in this case to the detriment of the luminous efficacy. If the ratio is above 25, however, the influence of the Ce on the color properties of the lamp is so small that these strongly resemble those of the known high-pressure sodium lamps. It was found to be desirable that the lamp should radiate light with a color temperature Tc of at least 3000 K, and preferably between 3500 K and 4500 K, if it is to be used for a motor vehicle headlamp. To increase the color temperature value achievable with NaIxe2x80x94CeI3, it is possible, for example, to add CaI2 and DyI3 to the ionizable filling, for example in molar percentages 47 Na, 7.7 Ce, 39.2 Ca, and 6.1 Dy.
Xe is added to the ionizable filling of the discharge vessel with a high filling pressure. The Xe here ensures a fast lumen output immediately after ignition of the lamp. The choice of the filling pressure of the rare gas in addition influences the heat balance of the discharge vessel, and thus the useful life of the lamp. It was found that a pressure of at least 5 bar is required for realizing a lamp life of 10,000 switching operations. Preferably, the filling pressure lies in a range from 7 bar to 20 bar, more in particular from 10 bar to 20 bar. This offers a possibility of realizing switching lives of 20,000 switching operations and more.